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Went on a service call last week that the homeowners were complaining about the upstairs H.P. was running continuously even with the t-stat in the off position. This is a 2 system house with a 1.5 ton upstairs and a 2.5 ton downstairs. When I first got there i noticed that the installers had the 2 heat pumps backwards ( 2.5 ton H.P. on 1.5 ton air handler and vice versa). So the first thing I did was switch the units around. after getting both units up and running both units operated right like they should never getting either unit to run with t-stat in off position like homeowner had stated. Even replaced t-stat upstairs and redid low voltage on the upstairs unit thinking that there might be a short in them which would cause this unit to run all the time. They call back 3 days later saying the unit had started doing the same thing, which is know the downstair H.P. I hope and think that we can assume that the problem is within the H.P. itself. Going back tommorrow to see what I can do, Any suggestions?

Was it frozen? If the contactor welded, unless the fan switch was set in the ON position, the system would freeze when the thermostat satisfied and the indoor blower shut off, leaving just the condenser running.

There is nothing more frightful than ignorance in action....Mark Twain